Ashing method for removing an organic film on a substance of a semiconductor device under fabrication

ABSTRACT

Plasma ashing methods, for moving a resist material formed on a ground layer of a semiconductor device during fabrication of said semiconductor, are performed by using one of three kinds of reactant gases each composed of three different gases. Plasma ashing is performed: at an ashing rate of 0.5 μm/min at 160° C. and with an activation energy of 0.4 eV when a reactant gas composed of oxygen, water vapor and nitrogen is used; at an ashing rate of 0.5 μm/min at 140° C., with an activation energy of 0.38 eV and without etching the ground layer when a reactant gas composed of oxygen, water vapor and tetrafluoromethane is used; and at an ashing rate of 0.5 μm/min at 140° C., with an activation energy of 0.4 eV when a reactant gas composed of oxygen, hydrogen and nitrogen is used.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of removing an organicmaterial used on a semiconductor device, and particularly the presentinvention relates to an ashing method of removing an organic filmtemporarily formed on a substrate of a semiconductor device duringfabrication.

An organic film, such as a resist or a polyamide film temporarily formedon a substrate, which is a part of a semiconductor device, as part ofthe process of fabricating the semiconductor device has in the past beenremoved by an ashing method using an oxygen plasma. Removing the resistfilm is an important part of the process of fabricating a semiconductordevice. Removing the resist film, as an organic film, will be describedhereinafter. Since the semiconductor device is very small as compared toa Large Scale Integrated circuit device (LSI) or a Very Large ScaleIntegrated circuit device (VLSI) in which it is used, the resist film,which will be called simply the "resist" hereinafter, is hard to remove,by the usual ashing method using the oxygen plasma, without damaging thedevices. During the process of ion implantation and dry etching, whichare widely used in process of fabricating process of LSIs or VLSIs, theproperties of the resist are substantially changed, causing the ashingrate of the resist to be slow, so that a long time is required to ashthe resist. Since resists are used many times in the process offabricating LSIs or VLSIs, the ashing rate for each resist should behigh in order to improve the throughput of the fabrication process.

A layer, called simply a "ground layer" hereinafter, upon which theresist is formed, is usually made of material, such as silicon dioxide(SiO₂ ), polysilicon (Si) or aluminum (Al). Generally, the resist is noteasily etched intentionally by the ashing method. In other words, theresist is hard to etch precisely so as to be able to remove only theresist without damaging any of the ground layer in the ashing process.Therefore, when an ashing method is used in the process of fabricatingLSIs or VLSIs, great attention must be paid to leaving the ground layeras it is, because the ground layer of the LSI or the VLSI is very thinand not even a small part of it can be permitted to be etched.

There are many kinds of plasma ashing methods for removing a resist filmwhich has been provided on an insulating layer in a semiconductor. Themost widely used is a down-flow ashing method because, applying thedown-flow ashing method to the plasma ashing process, damage caused bycharged particles can be avoided. The down-flow ashing rate generallydepends on the temperature, which will hereinafter be called the "ashingtemperature", of the resist, such that the ashing rate decreases with adecrease in the ashing temperature. The ashing rate is usually expressedby the well known Arrhenius plot by which the ashing rates are plottedin a line against the inverse numbers of the respective ashingtemperatures. In the Arrhenius plot, the gradient of the line gives theactivation energy for ashing such that, when the ashing rate decreasesrapidly with the decrease of the ashing temperature, the activationenergy is large, and such that, when the ashing rate changes only littlewith the decrease of the ashing temperature, the activation energy issmall. In this process, a small activation energy is desirable becauseashing can then be performed almost independently of the ashingtemperature. In other words, where the activation energy is small,ashing can be performed in a stable and precise manner

Recently, there has been a tendency to use a process for fabricatingsemiconductor devices performed at lower temperatures, in accordancewith the trend of miniaturization of devices as in the LSI or the VLSI.It is desirable to perform the ashing process at a temperature lowerthan 300° C., most preferably lower than 200° C., to avoid contaminationfrom the resist. In order to maintain a high ashing rate at such lowtemperatures, higher than 0.5μ/min for practical use, the activationenergy of the ashing rate must also be low. The activation energy of theashing rate can be changed to some extent by changing the particularreactant gases used for ashing. The selection of the reactant gases, andparticularly the use of combinations of such gases, is very important inorder to provide high rates and low activation energies, and toprecisely etch the resist, while leaving the ground layer as unchangedas possible and for minimizing damage. The selection and the combinationof the reactant gases has been studied energetically.

2. Description of the Related Art

Downflow ashing is performed in a downflow of microwave plasma using amicrowave plasma resist stripper. This is fully disclosed in a papertitled "Heavy Metal Contamination From Resists during Plasma Stripping"by Shuzo Fujimura and Hiroshi Yano, in Elect. Chem. Soc. Vol. 135, No.5, May 1988.

The downflow microwave plasma resist stripper comprises a vacuum chamberincluding a plasma generating chamber, a vacuum pump for exhausting gasin the vacuum chamber, a process chamber including a pedestal on which asample wafer is placed and a microwave power source. A reactant gas issupplied to the process chamber through the plasma generating chamber.

Then, a reactant gas plasma is generated in the plasma generatingchamber by microwaves, so that active species for ashing in the gasplasma proceed to flow down to the process chamber and react with aresist, which has previously been formed on the sample wafer, so as toremove the resist.

In the downflow ashing process, oxygen has long been used as thereactant gas as described before. However, when only oxygen is used, theashing rate is low and the activation energy is high, so that downflowashing, using only oxygen, was hard to apply to the process offabricating LSIs or VLSIs. Therefore, many other reactant gases havebeen studied as a means to increase the ashing rate and decrease theactivation energy. It has even been studied to combine other kinds ofgases with oxygen. As a result, several kinds of effective reactantgases have been found as will be described below, giving four examples,a first, a second, a third and a fourth examples, tracing thedevelopment of these new reactant gases. Hereinafter, the ashing rateand the activation energy are shown to be related to removing the resistfilm provided on a semiconductor device by the plasma ashing method.

A first reactant gas was a mixed gas of oxygen (O₂) with a halogenidegas, such as tetrafluoromethane (CF₄). The first reactant gas was mostcommonly used because it had a high ashing rate. FIG. 1 shows the ashingrate for commercially available photoresist (OFPR-800, TOKYO-OHKA)plotted against the variation of proportion of tetrafluoromethane in themixed gas, as measured by the flow rate of tetrafluoromethane to themixed gas at room temperature. Hereinafter, the ashing rate in the caseusing a particular reactant gas is simply called the ashing rate withthe reactant gas. In FIG. 1, when as much as 15% tetrafluoromethane isadded to oxygen, the ashing rate reaches a maximum value of 1.5 μm/minat 25° C., which is high enough for practical use. However, the groundlayer, such as SiO₂, polysilicon (Si) or Al, is also etched because offluorine (F) which is found mixed in the first reactant gas. On theother hand, when this first reactant gas is used, the activation energyis drastically reduced to a value of 0.1 eV from 0.52 eV, which is aboutthe same activation energy found when using only oxygen. Such largedecrease of the activation energy is due to the tetrafluoromethane,which was explained in the paper, J. J. Hannon and J. M. Cook, J.Electrochem. Soc., Vol. 131, No. 5, pp 1164 (1984).

A second reactant gas was a mixed gas of oxygen and nitrogen (N₂), notcontaining fluorine (F), which did not etch the ground layer. The ashingrate and the concentration of oxygen atom in the a down-flowed gas weremeasured by varying the flow ratio of nitrogen to the second reactantgas as shown in FIG. 2; wherein, the concentration of oxygen atom wasmeasured by an actinometry method. In this case, the ashing temperaturewas 200° C. and the flow rate of the second reactant gas was 1000Standard Cubic Centimeters per Minute (SCCM). In FIG. 2, white circlesrepresent the concentrations of oxygen atom, obtained from the spectralintensity ratio of the radiation from an oxygen atom (at a wavelength of6158 Å) to the radiation from an argon atom (at a wavelength of 7067 Å),and triangles represent the concentrations of the same oxygen atom,obtained from the spectral intensity ratio of the radiation from anoxygen atom (at a wave length of 4368 Å) to the radiation from an argonatom (at a wavelength of 7067 Å). Further, the values of theseconcentrations are normalized by a maximum of the values of theconcentrations, positioned at about 10% of the flow ratio of nitrogen tothe second reactant gas. Multiplication signs represent the ashing ratesat the respective flow ratios of nitrogen to the second reactant gas. Ascan be seen from FIG. 2, the curve of the ashing rate and that of theconcentration of oxygen atom coincide with each other, which means thatoxygen atoms are only effective in performing the ashing. FIG. 3 showsan Arrhenius plot of the ashing rate when the second reactant gascontains 90% of oxygen and 10% of nitrogen in the mixture and anArrhenius plot of the ashing as accomplished with oxygen gas only. Theashing temperature is denoted by T. The ashing rate of the secondreactant gas is plotted by a circle and the ashing rate of the oxygen isplotted by a multiplication sign. The ashing rate with the secondreactant gas is about two times of that of oxygen alone. Hereinafter,the activation energy of ashing, in the case of using a reactant gas, issimply called the activation energy of the reactant gas. The activationenergy (Ea) of the second reactant gas, and that of oxygen are equally0.52 eV. That is, the activation energy did not change by mixingnitrogen with oxygen. The ashing rate of the second reactant gas, of 0.2μm/min at 160° C., is too small for practical use. In order to increasethe ashing rate, another kind of gas was needed.

The third reactant gas was a mixed gas of oxygen and water vapor (H₂ O)which did not etch the ground layer. The ashing rate and concentrationof oxygen atoms were measured by varying the flow ratios of water vaporto the third reactant gas as shown in FIG. 4. The measurements wereperformed at 180° C. ashing temperature and 1000 SCCM flow rate of thethird reactant gas. Circles and multiplication signs in FIG. 4 representthe same as in FIG. 2, respectively. When the flow ratio of water vaporto the third reactant gas exceeded 40%, the concentration of oxygenatoms decreased with an increase of the water vapor flow ratio. However,the ashing rate did not decrease as much as the decrease of theconcentration of oxygen atom, as seen in FIG. 4. This means that someactive species, other than oxygen atoms, are possibly taking part in theashing. FIG. 5 compares the Arrhenius plot of the third reactant gas,containing 60% of oxygen and 40% of water vapor, and Arrhenius plot ofoxygen gas alone. The ashing rate of the third reactant gas, having 40%flow ratio of water vapor, is plotted by triangles and the ashing rateof the oxygen alone is plotted by multiplication signs. The activationenergy of the third reactive gas is 0.39 eV, which is about threequarters of the activation energy (0.52 eV) of oxygen alone, as shown inFIG. 6. FIG. 6 shows the activation energy of ashing in the case ofusing a third reactant gas by varying the flow ratios of water vapor tothe third reactant gas, by white circles. In FIG. 6, the activationenergy of ashing in the case using a mixed gas of oxygen and hydrogen,by varying the flow ratio of hydrogen to the mixed gas, is shown bysolid circles for the sake of comparison. FIG. 6 shows that theactivation energy is easily reduced by adding a little water vapor andthe activation energy is constant independent of the flow ratio of watervapor when the flow ratio of water vapor to oxygen exceeds 5%. Theactivation energy of the second reactant gas is also indicated by a dotchain line in FIG. 6 for comparison with the third reactant gas. It isseen from this comparison that the activation energy does not change byadding nitrogen to oxygen. Behavior similar to the mixed gas of oxygenand water vapor is seen for the mixed gas of oxygen and hydrogen. On theother hand, the ashing rate of the third reactant gas is about 0.22μm/min at 160° C., as seen in FIG. 5. It has been concluded that thevalue of the ashing rate of the third reactant gas is still too smallfor practical use.

The fourth reactant gas is a mixed gas of oxygen, nitrogen andtetrafluoromethane. The fourth reactant gas is disclosed in the Japaneselaid-open patent application, SHO 63-102232, titled "DRY ETCHINGAPPARATUS" by Mikio Nonaka. When the flow ratio of tetrafluoromethaneand nitrogen are in the range of 5 to 20% and 5 to 10% respectively, alarge ratio of the rate of etching a positive resist to the rate ofetching a ground layer is obtained without decreasing the ashing rate.However, the etching of ground layer cannot be avoided in this case.

A mixed gas made by adding as little as 0.2% of hydrogen to a mixed gasof oxygen, nitrogen and tetrafluoromethane is commercially availablefrom EMERGENT TECHNOLOGIES CO. (Phoenix 2320 NORD Photoresist Stripper).In this case, the hydrogen diluted by nitrogen is added in order toimprove matching with microwave power. That is, adding hydrogen to themixed gas is not for reduction of the activation energy. So, the mixedgas is essentially the same as the first reactant gas. In fact, it isalso known that the activation energy of the second reactant gas mixedwith hydrogen does not decrease until there has been added about 0.5% ofhydrogen.

As seen from the description of the first, second, third and fourthreactant gases, an ideal mixed gas, having a high ashing rate and lowactivation energy and never etching the ground layer, has not been foundalthough much has been studied on new reactant gases.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to improve the ashingprocess for removing an organic film formed on a ground layer of asemiconductor device during fabrication, such that the ashing rateincreases, the activation energy decreases and the ground layer is neveretched in the ashing process.

The above object is achieved by using as the reactant gas in the ashingprocess, one comprising at least three kinds of gases. The reactant gasis separated into two groups of mixed gases: one (a first group gas) ofwhich comprises at least oxygen and water vapor, and the other (a secondgroup gas) of which comprises at least oxygen and hydrogen. As is wellknown, the oxygen in each group gas is the main gas for performing theashing, however water vapor in the first group gas and hydrogen in thesecond group gas are mainly for decreasing the activation energy,increasing the ashing rate and avoiding the ground layer being etched incooperation with another third gas respectively added to the first groupgas and the second group gas. The third gas added to the first group gasis hydrogen, nitrogen, nitrogen oxide or halogenide, and that added tothe second group gas is water vapor, nitrogen, nitrogen oxide orhalogenide. The halogenide includes tetrafluoromethane (CF₄), chlorine(Cl₂), nitrogen trifluoride (NF₃), hexafluoroethane (C₂ F₆) andmethyltrifluoride (CHF₃). The flow rate of the water vapor and the thirdgas of the first group gas and that of the hydrogen and the third gas ofthe second group gas are controlled respectively.

In the first group gas including a third gas of nitrogen, the watervapor added to the oxygen has the same effect of that described inreference to FIG. 5, and the added nitrogen has the same effect of thatdescribed in reference to FIG. 3. However, in the first group gasincluding a third gas of nitrogen, the added water vapor and nitrogenproduce a synergistic effect of increasing the ashing rate anddecreasing the activation energy. That is, it was simply assumed fromFIGS. 3 and 5 that the ashing rate of the first group gas, including thethird gas of nitrogen, would increase in the same way as the case of agas of only mixed oxygen and nitrogen, and the activation energy of thefirst group gas, including a third gas of nitrogen, would increase inthe same way as the case of a gas of only mixed oxygen and nitrogen, andthe activation energy of the first group gas, including a third gas ofnitrogen, would decrease in the same way as the case of a gas of onlymixed oxygen and water vapor. However, actually, the ashing rateincreased as much as two times the value to be assumed from FIG. 3though the activation energy only decreased to a value approximatelysame as the value assumed from FIG. 5.

The first group gas, including a third gas of halogenide such astetrafluoromethane, is excellent as a reactant gas. Recently, halogenidehas not been used as the third gas in the fabrication of LSIs or VLSIsbecause of the disadvantage that halogen easily etches the ground layer.However, according to experiments made by the inventors, it has foundthat the first group gas, including a third gas of halogenide,surprisingly does not etch the ground layer when water vapor is includedin the reactant gas, supplied by a flow ratio controlled so as to exceeda designated value. While it is not known why this takes place, it isbelieved that it is because the halogen atoms appear to react with thehydrogen atoms of the water molecules. As the result, the action of thehalogen is suppressed. Using the first group gas, including a third gasof halogen as the reactant gas, the ashing rate becomes large and theactivation energy of the ashing rate becomes small, compared with afirst group gas which includes a third gas of nitrogen.

As a second group gas, one including a third gas of nitrogen isdiscussed. According to experiments by the inventors, the activationenergy is decreased to 0.44 eV from 0.52 eV (the activation energycorresponding to oxygen) by adding more than 3% hydrogen. This isbecause the hydrogen acts to decrease the activation energy. The ashingrate of the second group gas, including as a third gas nitrogen, is asmuch as two or three times higher than that of the usual mixed gas ofoxygen and nitrogen.

These mixed gases, described above, of a first group gas includingnitrogen as the third gas, a first group gas including halogenide as thethird gas, and a second group gas, have three advantages of having highashing rates, having low activation energies and causing no ground layeretching in the process of fabricating LSIs or VLSIs.

BRIEF DESCRIPTION ON THE DRAWINGS

FIG. 1 is a graph showing the variation of the ashing rate for resist inthe case of using a reactant gas composed of O₂ and CF₄ as a function ofthe flow ratio of CF₄ to the reactant gas, at 25° C. ashing temperature.

FIG. 2 is a graph showing the variation of the ashing rate for resist inthe case of using a reactant gas composed of O₂ and N₂ at 200° C. ashingtemperature and the variation of the concentration of oxygen atom in adown-flowed gas with the flow ratio of N₂ to the reactant gas.

FIG. 3 is a graph showing Arrhenius plots of the ashing rate for resistin the case of using a reactant gas composed of 90% O₂, 10% N₂ and areactant gas composed of oxygen.

FIG. 4 is a graph showing the variation of the ashing rate in the caseof using a reactant gas composed of O₂ and H₂ O at 180° C. ashingtemperature and the variation of the concentration of oxygen atom in adown-flow gas with a flow ratio of H₂ O to the reactant gas.

FIG. 5 is a graph showing Arrhenius plots of the ashing rate for thecase of using a reactant gas composed of 60% O₂, 40% H₂ O and a reactantgas composed of oxygen.

FIG. 6 is a graph showing the variation of the activation energy ofashing using a reactant gas composed of O₂ and H₂ O with a flow ratio ofH₂ O to the reactant gas.

FIG. 7 is a schematical drawing of a vacuum chamber of a down-flowashing apparatus.

FIG. 8 is a graph of Arrhenius plots showing the ashing rates for resistin the case of using different kinds of reactant gases.

FIG. 9 is a graph showing the variations of the ashing rate in the caseof using a reactant gas composed of O₂, H₂ O and N₂ at 180° C. and 200°C. ashing temperatures with a flow ratio of N₂ to the mixed gas of O₂and N₂, where the flow rate of H₂ O is kept constant.

FIG. 10 is a graph showing variations of the rate of etching SiO₂ groundlayer at 150° C. and 25° C. ashing temperature with flow ratio of H₂ Oto the mixed gas, in the case of performing plasma ashing using areactant gas composed of O₂, H₂ O and CF₄ at 25° C., 150° C. and 180° C.ashing temperature with a flow ratio of H₂ O to the reactant gas, where15% flow ratio of CF₄ to the mixed gas of O₂ and CF₄ is used.

FIG. 11 is a graph showing the variations of the ashing rate for resistin the case of using the reactant gas composed of O₂, H₂ O and CF₄ at25° C., 150° C. and 180° C. ashing temperature with flow ratio of H₂ Oto the reactant gas, where 15% flow ratio of CF₄ to the mixed gas of O₂and CF₄ is kept.

FIG. 12 is a graph showing Arrhenius plots of the ashing rate for resistin the case of using a reactant gas composed of O₂, H₂ and N₂, areactant gas composed of O₂ and N₂ and a reactant gas composed ofoxygen.

FIG. 13 is a graph showing variations of the activation energy of ashingusing a reactant gas composed of O₂ and H₂ O and a reactant gas composedof O₂ and H₂, with a flow ratio of H₂ O to the reactant gas of O₂ and H₂O and with a flow ratio of H₂ to the reactant gas of O₂ and H₂,respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The plasma ashing methods of removing resist, using three kinds ofreactant gases will be described as the embodiments of the presentinvention with reference to FIGS. 8 to 12. The embodiments are separatedinto three, a first, a second and a third embodiment, as a function ofthe three kinds of the reactant gases. Plasma ashing was performed by adown flow ashing method using a conventional downflow microwave plasmaresist stripper schematically shown in FIG. 7.

THE FIRST EMBODIMENT

The first embodiment is the plasma ashing method using a reactant gascomposed of O₂, H₂ and N₂. In FIG. 7, the reactant gas composed of 720SCCM O₂, 100 SCCM H₂ O and 180 SCCM N₂ is supplied to an initiallyexhausted vacuum chamber 6 through a gas inlet 3, keeping the gaspressure in the vacuum chamber 6 at about 0.8 Torr. The flow rates ofthe gases of O₂, H₂ and N₂ are controlled respectively by a controller,not depicted, before the gases flow into the vacuum chamber 6. Through awave guide 1 and a window 2, microwave power of 2.45 GHz is suppliedinto a plasma generating chamber 4. Plasma is generated with thereactant gas in the plasma generating chamber 4 and the chargedparticles in the plasma are trapped by a shower plate 5. Then, onlyneutral active species, generated in the plasma, flow down to a processchamber 4' through the holes provided in a showerhead plate 5 and toucha surface of a resist 10 formed on a ground layer 9 of a sample wafer 8placed on a stage 7 heated to 140° C. by a heater, not depicted. As aresult, the resist 10 is etched until the surface of the ground layer 9appears while not etching any part of the ground layer. Because thereactant gas does not contain halogen, the ground layer 9 is neveretched.

The ashing rate, in the case of using a reactant gas composed of O₂, H₂and N₂, is shown by a line connecting open triangles in FIG. 8representing an Arrhenius plot. In FIG. 8, the ashing rates in the caseof using several other kinds of reactant gases are also shown in thesame way, for the sake of comparison. That is, the ashing rate in thecase of using a reactant gas composed of only O₂, that of using areactant gas composed of O₂ and H₂ O and that of using a reactant gascomposed of O₂ and N₂ are shown by solid circles, solid triangles andopen circles, respectively. As seen in FIG. 8, the ashing rate, in thecase of using a reactant gas of O₂, H₂ O and N₂, is larger than theashing rate in the cases of using a reactant gas composed of only O₂, ofO₂ and H₂ O, or of O₂ and N₂. The reactant gas composed of O₂, H₂ and N₂has an ashing rate of 0.5 μm/min at 160° C. ashing temperature and anactivation energy of 0.4 eV. For practical use, the 0.5 μm/min ashingrate is large enough and the 0.4 eV activation energy is small enough.FIG. 9 shows the variation of the ashing rate for the various flowratios of N₂ to the mixed gas of O₂ and N₂, at an ashing temperatures of180° C. and 200° C., keeping the flow rate of H₂ O at 100 SCCM and thetotal flow rate of O₂ and N₂ at 900 SCCM. It can be seen in FIG. 9 thatthe ashing rate in the case of using a reactant gas composed O₂, H₂ Oand N₂ is hardly changed by varying the flow ratio of nitrogen to themixed gas of O₂ and N₂ when the flow ratio is larger than 5%. Therefore,the reactant gas of mixed O₂, H₂ O and N₂, having a nitrogen flow ratiolarger than 5%, can be also used as the reactant gas for performing aprecise ashing process.

The flow rate of each component gas is set at 720 SCCM, 100 SCCM and 180SCCM for O₂, H₂ O and N₂, respectively, as a desirable example. However,the flow rate of N₂ is not limited if the flow ratio of N₂ to the mixedgas of O₂ and N₂ is larger than 5%, because the ashing rate is constantregardless of the nitrogen flow ratio when it exceeds 5% as seen in FIG.9. The flow rate of H₂ O is not limited if the flow ratio of H₂ O to themixed gas of O₂ and H₂ O is larger than 1% because the activation energyis about 0.4 eV regardless of the water flow ratio when its flow ratioexceeds 1%, as seen in FIG. 6.

In the first embodiment, NO_(x) or H₂ can be added to mixed gas of O₂and H₂ O instead of N₂.

THE SECOND EMBODIMENT

In the second embodiment, a reactant gas composed of O₂, H₂ O and CF₄ isused for the plasma ashing. The ashing is performed in the same way asdescribed in the first embodiment. The flow rates of O₂, H₂ O and CF₄are 730 SCCM, 150 SCCM and 120 SCCM, respectively.

The disadvantage of etching the ground layer by using CF₄ can be avoidedby maintaining the flow ratio of H₂ O to the reactant gas larger than10% as shown in FIG. 10. FIG. 10 is a graph showing the etching rate, at150° C. and 25° C. ashing temperature, of a ground layer made of SiO₂,with a flow ratio of H₂ O to the reactant gas, under a condition thatthe total flow rate of O₂, H₂ O and CF₄ is kept at 1000 SCCM and theflow rate of CF₄ to the mixed gas of O₂ +CF₄ is kept at 15%. From FIG.10, it can be seen that when the flow ratio of H₂ O to the reactant gasexceeds 10%, the etching rate of the ground layer of SiO₂ becomes zero,in other words, the ground layer of SiO₂ is never etched. Thus, this 10%of H₂ O ratio is a very important percentage in order to avoid theground layer being etched. Incidentally, when the H₂ O ratio to thereactant gas is 10%, it can be calculated from the above condition inFIG. 10, that the flow ratios of O₂ and CF₄ to the reactant gas are76.5% and 13.5% respectively.

The ashing rate of a reactant gas composed of O₂, H₂ O and CF₄ is shownby a line connecting reversed solid triangles in FIG. 8. As shown inFIG. 8, the ashing rate is the largest, compared with other ashingrates, in the cases of using other reactant gases.

The ashing rate of a reactant gas composed of O₂, H₂ O and CF₄ wasmeasured by varying the flow ratio of H₂ O to the reactant gas at 25°C., 150° C. and 180° C. ashing temperatures respectively, under acondition that the total flow rate of O₂, H₂ O and CF₄ was kept at 1000SCCM and the flow ratio of CF₄ to the mixed gas of O₂ and CF₄ was keptat 15%. The results of the measurement are shown in FIG. 11, where itcan be seen that when the flow ratio of H₂ O to the reactant gas exceeds10%, the ashing rate is constant and independent of flow ratio of H₂ Oto the reactant gas.

The flow rate of each component gas is set at 730 SCCM, 150 SCCM and 120SCCM for O₂, H₂ O and CF₄, respectively, as a desirable example. Howeverthe flow rate of H₂ O is not limited if the number of hydrogen atomsderived from H₂ O is larger than the number of fluorine atoms liberatedfrom CF₄. On the other hand, the flow ratio of H₂ O to the mixed gas ofO₂ and H₂ O must be larger than 1%, so that the activation energy ofashing in the case of using the reactant gas is about 0.4 eV regardlessof the flow ratio, as seen in FIG. 6.

In the second embodiment, CF₄ is used as a halogenide, however Cl₂, NF₃,C₂ F₆ and CHF₃ can be used instead of CF₃.

THE THIRD EMBODIMENT

The third embodiment is a plasma ashing method using a reactant gascomposed of O₂, H₂ and N₂. That is, the reactant gas composed of 720SCCM O₂, 100 SCCM H₂ and 180 SCCM N₂ is applied to the downflow ashingprocess as described in the first embodiment.

In the case of using a reactant gas composed of O₂, H₂ and N₂, theashing rate is shown by a line connecting open squares in FIG. 12, whichrepresents an Arrhenius plot, and the activation energy is shown with aflow rate of H₂ to the reactant gas composed of O₂, H₂ and N₂ in FIG.13.

In FIG. 12, the ashing rate, in the case of using a reactant gascomposed of only O₂ and that of using a reactant gas composed of O₂ andN₂, are also shown by a line connecting multiplication signs and a lineconnecting open circles respectively, for the sake of the comparisonwith a reactant gas composed of O₂, H₂ and N₂. In the case of using thereactant gas composed of O₂, H₂ and N₂, the ashing rate is about 0.7μm/min at 160° C. ashing temperature as shown in FIG. 12, and theactivation energy is about 0.4 eV as shown in FIG. 13.

From consideration of FIG. 13, it is clear that the activation energyrapidly decreases to a value of approximately 0.4 eV as the flow ratiois increased from 0% to approximately 5% and stays constantly at a valueof about 0.4 eV in a region where the flow ratio is higher that 5%.Plasma ashing is actually carried out in this constant region ofactivation energy. In FIG. 13, it can be seen that the characteristic ofhaving substantially constant activation energy is very important forperforming plasma ashing in a stable and precise manner.

The flow rate of each component of the gas is set at 720 SCCM, 100 SCCMand 180 SCCM for O₂, H₂ and N₂, respectively. However, if the flow rateof the H₂ in the reactant gas is greater than 3%, the flow rate of thehydrogen is not limited because the activation energy of ashing, wherethis reactant gas is used, is about 0.4 eV regardless of flow rate , asseen in FIG. 13. The flow rate of N₂ is not limited if the flow rate ofN₂ to the mixed gas of O₂ and N₂ is larger than 5%, because the ashingrate is constant, regardless of the flow rate, when the flow rateexceeds 5%, as seen in FIG. 9.

In the third embodiment, plasma ashing is performed by using a reactantgas of O₂, H₂ and N₂. However, it was confirmed that H₂ O, NO_(x) orhalogenide can be used instead of N₂.

The first, second and third embodiments described above are related toremoving the resist film by plasma ashing, however the present inventioncan be applied to removing any organic polymer film.

Though the reactant gases described in the first, second and thirdembodiments are composed of three kinds of gases, an inert gas, such asHe, Ne or Ar, can be added to the reactant gases up to 7%.

What is claimed is:
 1. A method for removing an organic material formedon a ground substance of a semiconductor device, comprising the stepsof:providing a mixed reactant gas comprising oxygen, water vapor and anadditional gas in a proportion relative to each other such that uponforming said mixed gas into a plasma and impinging said plasma upon asurface containing said organic material to be ashed under ashingconditions, the ashing rate is higher than it would be if the reactantgas was composed of only oxygen and water and the activation energy islower than it would be if the reactant gas was composed of only oxygenand water vapor; generating a first plasma of said mixed reactant gasproviding a second plasma down stream of said first plasma for producingactive species thereof; and impinging said second plasma on said organicmaterial down stream from the making of said first plasma for a time andunder condition sufficient to ash said organic material but insufficientto attack said ground substance.
 2. A method according to claim 1,wherein said additional gas is at least one selected from the groupconsisting of hydrogen, nitrogen, nitrogen oxide, tetrafluoromethane,chlorine, nitrogen trifluoride, hexafluoroethane and trifluoromethane.3. A method according to claim 2, wherein the additional gas is nitrogenand the ratio of water vapor to the sum of water vapor and oxygen ismore than 1% and the ratio of nitrogen to the sum of nitrogen oxygen ismore than 5%.
 4. A method according to claim 3 wherein the temperatureof the organic material is higher than 160° C. so as to obtain an ashingrate higher than 0.5 μm/min.
 5. A method according to claim 2, whereinthe additional gas is tetrafluoromethane in a ratio to the water vaporsuch that the number of hydrogen atoms in the water is at least as greatas the number of halogen atoms in the tetrafluoromethane, and the ratioof water vapor to the sum of the water vapor and the oxygen is more than1%.
 6. A method according to claim 5 wherein the additional gas isselected from a halogenide group consisting of tetrafluoromethane,chlorine, nitrogen trifluoride, hexafluoroethane and trifluoromethane.7. A method according to claim 5 wherein the temperature of the organicmaterial is higher than about 140° C., so as to obtain an ashing ratelarger than 0.5 μm/min.